Re: [PATCH bpf-next] selftests/bpf: Fix flaky test ptr_untrusted

[Yonghong Song <yonghong.song@linux.dev>]

On 2/5/24 10:56 AM, Andrii Nakryiko wrote:
> On Sun, Feb 4, 2024 at 11:45 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>> Somehow recently I frequently hit the following test failure
>> with either ./test_progs or ./test_progs-cpuv4:
>>    serial_test_ptr_untrusted:PASS:skel_open 0 nsec
>>    serial_test_ptr_untrusted:PASS:lsm_attach 0 nsec
>>    serial_test_ptr_untrusted:PASS:raw_tp_attach 0 nsec
>>    serial_test_ptr_untrusted:FAIL:cmp_tp_name unexpected cmp_tp_name: actual -115 != expected 0
>>    #182     ptr_untrusted:FAIL
>>
>> Further investigation found the failure is due to
>>    bpf_probe_read_user_str()
>> where reading user-level string attr->raw_tracepoint.name
>> is not successfully, most likely due to the
>> string itself still in disk and not populated into memory yet.
>>
>> One solution is do a printf() call of the string before doing bpf
>> syscall which will force the raw_tracepoint.name into memory.
>> But I think a more robust solution is to use bpf_copy_from_user()
>> which is used in sleepable program and can tolerate page fault,
>> and the fix here used the latter approach.
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   tools/testing/selftests/bpf/progs/test_ptr_untrusted.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/progs/test_ptr_untrusted.c b/tools/testing/selftests/bpf/progs/test_ptr_untrusted.c
>> index 4bdd65b5aa2d..2fdc44e76624 100644
>> --- a/tools/testing/selftests/bpf/progs/test_ptr_untrusted.c
>> +++ b/tools/testing/selftests/bpf/progs/test_ptr_untrusted.c
>> @@ -6,13 +6,13 @@
>>
>>   char tp_name[128];
>>
>> -SEC("lsm/bpf")
>> +SEC("lsm.s/bpf")
>>   int BPF_PROG(lsm_run, int cmd, union bpf_attr *attr, unsigned int size)
>>   {
>>          switch (cmd) {
>>          case BPF_RAW_TRACEPOINT_OPEN:
>> -               bpf_probe_read_user_str(tp_name, sizeof(tp_name) - 1,
>> -                                       (void *)attr->raw_tracepoint.name);
>> +               bpf_copy_from_user(tp_name, sizeof(tp_name) - 1,
>> +                                  (void *)attr->raw_tracepoint.name);
> Should we also add bpf_copy_from_user_str (and
> bpf_copy_from_user_str_task) kfuncs to complete bpf_copy_from_user?
> This change is not strictly equivalent (though for tests it's fine,
> but in real-world apps it would be problematic).

Sounds a good idea. Let me do some investigations!

>
>>                  break;
>>          default:
>>                  break;
>> --
>> 2.34.1
>>